Magnetic resonance (MR) imaging was performed on 21 presumed normal Beagle-type dogs. The size and symmetry of their lateral ventricles were evaluated and dogs were categorized on the basis of the percentage of their ventricular height (Vh) to brain height (Bh) and the ratio of the largest to the smallest ventricular area (rVA). Eleven dogs had lateral ventricles classified as normal sized (0-14% Vh/Bh) while 10 of 21 dogs had moderate enlargement (15-25% Vh/Bh) of one or both lateral ventricles. None of the dogs had severe lateral ventricular enlargement. Degree of asymmetry was also arbitrarily categorized on basis of rVA as normal to minimal (rVA < 1.5), mild (1.5 < rVA < 2.0), or severe (2.0 < rVA). Of the dogs having normal-sized lateral ventricles, six of eleven had symmetric, three of eleven had mildly asymmetric and two of eleven had severely asymmetric lateral ventricles. Of the dogs having at least one moderately enlarged lateral ventricles, five of ten had symmetric lateral ventricles, and two of ten had mild asymmetry and three of ten had severe asymmetry. Gender and body weight had no statistical relationship to lateral ventricle symmetry. Clinically insignificant ventricular enlargement and asymmetry was common in this group of Beagle dogs.

Eighteen healthy Beagle-type dogs were studied using magnetic resonance (MR) imaging from a few days after birth up to 7 months of age. We evaluated the onset of lateral ventricular expansion, the developmental change of lateral ventricular volume and the ratio of the largest to the smallest lateral ventricular volume. The onset of lateral ventricular expansion was defined as the day that the expansion by cerebrospinal fluid (CSF) was first visible in unilateral or bilateral lateral ventricles on the transverse images at the level of the intraventricular foramen. It was found that the expansion of lateral ventricles were first detectable at 3-4 weeks. Lateral ventricular volume ratio varied most from the onset of lateral ventricular expansion to 75 days of age and stabilized after that, although absolute brain and lateral ventricular volumes continued to increase.

Key word: absence of septum pellucidum, dog, MRI.